“They do not believe their’s is the only true denomination, they believe that Christ set up one true Church, and their’s is it.”
I am sorry but most “Protestants” - Methodists, Episcopalians, Presbyterians, UCC and the like DO NOT believe that “their” church is the “one true Church”.
They accept that most of their doctrinal differences are less than what they hold in common belief, even while they prefer to practice their Christian beliefs in the framework of their own denominations; and they do not - most of them - hold that someone is NOT in the “body of Christ” just because they are part of another denomination.
Some groups commonly classified as Protestant do indeed believe this.
I am not trying to say that every Protestant group, or even most, believe this. Only those Protestant churches which claim the others have fallen away.
One common definition, perhaps the most commonly used, for Protestant is all "Christian" groups other than Catholic, Orthodox and those groups such as Mormons and JWs which explicitly exclude themselves from the Protestant grouping.